A description will be given of a conventional wavelength division multiplexed (hereinafter referred to as WDM) optical signal transmission system with reference to FIG. 1.
As shown in FIG. 1, the conventional WDM optical signal transmission system comprises a wavelength multiplexer 100 as an upstream device, an optical repeater 900, and a wavelength demultiplexer 102 as a downstream device. The WDM optical signal transmission system is provided with optical fibers 7-1 and 7-2 in the route from the wavelength multiplexer 100 to the wavelength demultiplexer 102 to transmit WDM optical signals.
There may be disposed a plurality of the optical repeaters 900 depending on the length of a transmission line or the attenuation in the level of a received optical signal. The optical repeater 900 is capable of diverting an arbitrary optical signal to an external interface as well as wavelength multiplexing optical signals newly received from the external interface.
In FIG. 1 illustrating the wavelength multiplexer 100, optical repeater 900, and wavelength demultiplexer 102, the reference numeral 1-1 represents external interface signal receiving sections. Each of the external interface signal receiving sections 1-1 receives an optical signal from an opposite device and converts it to a signal having an arbitrary wavelength. Any device may serve as the opposite device so long as it deals with signals that can be interfaced with the external interface signal receiving section 1-1 and an external interface signal transmitting section 6-1. Examples of the opposite device include an STM (Synchronous Transfer Mode) transmitter dealing with STM signals, an ATM (Asynchronous Transfer Mode) transmitter, and a router dealing with gigabit signals on the Internet, etc. The wavelength multiplexer and demultiplexer may constitute opposite devices. The wavelength multiplexer and demultiplexer do not controls respective interface signals and restore their wavelengths to their original conditions when outputting them. The reference numerals 2-1 and 2-2 represent wavelength multiplexing sections. Having received optical signals of n wavelengths λ1 to λn obtained by the external interface signal receiving sections 1-1, the wavelength multiplexing section 2-1 or 2-2 generates a WDM optical signal and adjust the optical level according to the number of the wavelengths to send it to the transmission line. Further, the reference numerals 3, 3-1 and 3-3 represent wavelength number information managing sections for managing the number of wavelengths of optical signals wavelength multiplexed into a WDM optical signal.
“The number of wavelengths” (also referred to as the total number of wavelengths) herein indicates the number of optical signals multiplexed into a WDM optical signal. For example, when 16 optical signals are multiplexed, optical signals of 16 different wavelengths are multiplexed. Similarly, when 40 optical signals are multiplexed, optical signals of 40 different wavelengths are multiplexed. Besides, “wavelength” sometimes indicates each of optical signals multiplexed into a WDM optical signal. Respective optical signals are converted to signals having arbitrary wavelengths, and there exist no optical signals having the same wavelength. Further, “transmission” does not mean radio transmission, but it means the transmission of electrical signals through an internal bus or the transmission of a WDM signal through an optical fiber.
The reference numerals 4-1 and 4-2 represent wavelength demultiplexing sections. Having received a WDM optical signal from the transmission line, the wavelength demultiplexing section 4-1 or 4-2 adjusts the optical level according to the number of the wavelengths and demultiplexes the signal into signals having different wavelengths. The reference numeral 5-1 represents wavelength repeating sections for compensating a deteriorated signal to restore it to its original quality. The reference numeral 6-1 represents an external interface signal transmitting section. The external interface signal transmitting section 6-1 transmits a signal to an opposite device. The reference numerals S1 and S2 represent WDM optical signals.
In, for example, Japanese Patent Applications laid open No. HEI10-303823 and No. 2001-7829, there have been described techniques for transmitting information on the number of wavelengths with such devices as the wavelength multiplexer 100, optical repeater 900, and wavelength demultiplexer 102.
The conventional techniques, however, have the following problems.
The first problem is that, the information on the number of wavelengths (hereinafter referred to as wavelength number information), which is sent from the wavelength multiplexer to the optical repeater, indicates only the total number of wavelengths. Besides, the wavelength number information is simply created based on input levels in each single device regardless of settings for respective wavelengths.
The second problem is that, when a plurality of optical repeaters are connected in series as shown in FIG. 2, or intervals between wavelengths to be multiplexed are narrow, an error occurs in detection.
FIG. 2 is a diagram showing a plurality of optical repeaters connected in series.
Particularly, in such a system, with a plurality of optical repeaters 900-1, 900-2, . . . , 900-n being connected in series, as shown in FIG. 2, an error easily occurs in detection. Therefore, a system capable of preventing erroneous detection is desired.
FIG. 3(a) is a diagram showing the row of wavelengths to be multiplexed. FIG. 3(b) is a diagram showing a part depicted in FIG. 3(a) on larger scale. In both the drawings, the horizontal axis indicates wavelength (nm) while the vertical axis indicates optical signal level (dB).
As can be seen in FIG. 3(b), in the case of the system comprising a plurality of the optical repeaters connected in series, the waveform of a WDM optical signal distorts as indicated by the dotted line N, which causes optical noise in a WDM optical signal. Provided that the intensity of optical noise is Ln, an increase in Ln causes a false representation of an optical signal with a wavelength (λX in FIG. 3(b)), at which originally no signal is present, when a WDM optical signal is demultiplexed. Thus, the number of wavelengths may be erroneously detected.
That is, according to the conventional techniques, since the wavelength number information is created based on only the presence or absence of optical signals to be wavelength multiplexed, the number of wavelengths is erroneously detected when optical noise is caused in the optical repeater. Consequently, such functions as adjustment and monitoring of the level of a WDM optical signal can be performed erroneously or unusually.